This invention relates to power generation apparatus containing fuel cells and particularly, but not exclusively, to apparatus which allow the co-production of hydrogen as well as electricity.
There is an ever increasing need to produce power as efficiently and cleanly as possible. Of particular concern is the discharge of carbon dioxide into the atmosphere. This is widely recognised to contribute to global warming and thus efforts are made to reduce carbon dioxide emissions into the atmosphere. One way of achieving this is of course to increase the efficiency with which power is generated from fuel. Another potential way of reducing carbon dioxide emissions into the atmosphere is to capture and store the carbon dioxide produced by the power generation process. This is problematic in conventional power generation systems based on combustion in air, however, since the carbon dioxide in the combustion products is mixed with a large amount of nitrogen. The presence of nitrogen makes the capture and separation of carbon dioxide significantly more expensive.
In recent years there has been a lot of interest in fuel cells which are devices which are able to generate an electric current and heat directly from fuel without combustion. The direct generation of electric current means that the efficiency of such devices is not limited by thermodynamic efficiency limits. However, power generation systems based on fuel cells may still produce carbon dioxide.
Most fuel cells operate on gaseous fuel, usually hydrogen (H2), methane (CH4) or carbon monoxide (CO), as well as oxygen (O2). A fuel cell comprises an anode and a cathode separated from each other by an electrolyte. Two types of electrochemical reactions occur: oxidation half-reaction(s) at the anode and reduction half-reaction(s) at the cathode. Typically, hydrogen (which may have been produced in situ from natural gas or other fuel in a process known as “reforming”) undergoes electrochemical reaction at the anode, oxygen (which may be supplied in the form of air) undergoes electrochemical reaction at the cathode and the net reaction provides water and generates electrical power. Other components, such as methane or carbon monoxide, may also be present in the inflow to the fuel cell, particularly where the hydrogen is prepared by natural gas steam-reforming or coal gasification. This means that as well as water, there may be other products such as carbon dioxide.
There are several types of fuel cell, some of which are described below.
PEM (Polymer Electrolyte Membrane or Proton Exchange Membrane) cells operate at low temperatures (60-100° C.). The electrolyte is a solid, flexible polymer. Hydrogen cations pass from the anode to the cathode. Platinum catalysts are used on both the cathode and anode. Water is produced at the cathode.
PAFC (Phosphoric Acid Fuel Cells) operate at moderate temperatures (150-200° C.). The electrolyte is a phosphoric acid matrix. Hydrogen cations pass from the anode to the cathode. Platinum catalysts are used on both the cathode and anode. A small amount of carbon monoxide in the hydrogen in-flow may be tolerated. Water is produced at the cathode. The reactions for both PEM fuel cells and PAFCs are:At anode: 2H2→4H++4e−At cathode: O2+4H++4e−→2H2ONet reaction: 2H2+02→2H20
MCFC (Molten Carbonate Fuel Cells) operate at high temperatures (600-1000° C.). The electrolyte is a matrix of carbonates (e.g. Lithium, Sodium, Potassium and/or Magnesium carbonates). Carbonate anions pass from the cathode to the anode, and carbonate anions lost in this way are compensated for by supplying carbon dioxide to the cathode. Carbon monoxide may also be present in the hydrogen in-flow and used as fuel. Water is produced at the anode. The reactions are:At anode: 2H2+2CO32−→2H2O+2CO2+4e−(also, if CO present: 2CO+2CO32−→4CO2+4e−)At cathode: O2+2CO2+4e−→2CO32−Net reaction: 2H2+02→2H20(also, if CO present: 2CO+O2→2CO2)
SOFC (Solid Oxide Fuel Cells) also operate at high temperatures (600-1000° C.). The electrolyte is a solid ceramic compound, e.g. zirconium oxides. Oxide ions pass from the cathode to the anode. Carbon monoxide may again be used as fuel. Water is produced at the anode. The reactions are:At anode: 2H2+2O2−→2H2O+4e+(also, if CO present: 2CO+2O2−→2CO2+4e−)At cathode: O2+4e−→2O2−Net reaction: 2H2+02→2H2O(also, if CO present: 2CO+O2→2CO2)
The two most promising types of fuel cell are the Solid Oxide Fuel Cell (SOFC) typically operating at 600-1000° C., and the Proton Exchange Membrane (PEM) fuel cell typically operating at 80° C.
The SOFC may operate on most gaseous hydrocarbon fuels or fuels derived from the reforming of natural gas, diesel, gasoline or the gasification of solid fuels. When carbonaceous fuels are used, the product gases will contain carbon dioxide. In stationary applications the carbon dioxide may be captured and sequestrated, but this is more difficult to realise in mobile applications like cars. Furthermore, the PEM fuel cell most commonly used for mobile applications generally requires purified hydrogen for operation below 150° C.
There remains a need for highly efficient and clean power and hydrogen generation systems to allow for a transition to a sustainable, low pollution use of fossil fuel energy without release of carbon dioxide to the atmosphere.
The separation of carbon dioxide may be realised by different means. One possibility is to use membranes to separate the different species, another is to absorb gases in liquids or solids and desorb the gases separately using pressure swing or temperature swing cycles.
Even though the efficiency of fuel cells is not limited thermodynamically, practically it has proven difficult to achieve efficiencies that approach the theoretical maximum. A number of hybrid systems have utilised the excess heat generated by an SOFC in a turbine or other machinery. However, these systems are complex, and the total efficiency is limited by the thermodynamic machinery.
One proposal is disclosed in U.S. Pat. No. 5,079,103. This document recognizes that hydrogen may be present in the gases exiting the anode of fuel cells such as MCFCs or SOFCs and, rather than using this for low-quality power generation (e.g. by combustion), seeks to separate it and carbon dioxide from the waste gas and utilize it more efficiently. The hydrogen may be separated from the waste gas by means of pressure swing absorption (PSA) and recycled back to the anode. The carbon dioxide may also be separated from the anode waste gas and, in the case of MCFCs, channelled to the cathode, thereby reducing the requirement for externally supplied carbon dioxide. The carbon dioxide may be separated from the anode waste gas by a scrubbing step or by PSA. Thus, this document discloses that recycling the hydrogen and carbon dioxide in this way provides more energy than simply burning the anode waste gas. The document also discloses, as anode feed stream, natural gas which is internally reformed to hydrogen. Because natural gas is used, desulphurization may be required and recycling the hydrogen can assist with this. However, although this document claims overall efficiencies of up to 70%, this is still some way below the theoretical maximum. Furthermore, because carbon dioxide removal is carried out on the outflow from the fuel cell, large volumes of gas have to be cleaned.
US 2001/0010873 discloses an SOFC wherein a hydrocarbon-containing fuel is introduced to a fuel cell and converted therein to a synthesis gas (an endothermic reaction). The synthesis gas then undergoes partial electrochemical reaction (an exothermic reaction) thereby generating electricity. The hydrocarbon-containing fuel is supplied in such an excess that no additional cooling of the fuel cell is required, i.e. production of the synthesis gas is sufficiently endothermic to counter-balance the exothermic electrochemical reaction. This document teaches using natural gas to which water has been added as the hydrocarbon-containing fuel. The conversion of methane and water to hydrogen and carbon dioxide occurs within the fuel cell before the electrochemical reaction. The process of US 2001/0010873 suffers from several problems. Firstly, it does not satisfactorily address the problem of efficient use of fuel. Secondly, it does not provide an efficient process for separation of the exhaust material. Thirdly, in order to avoid coking, the amount of oxygen must be kept low and this limits the electrochemical potential. This results in below optimum performance with respect to electrical efficiency and power density.
WO 02/15295 discloses a fuel cell generator in which the depleted fuel from a first fuel cell chamber is further used in a second fuel cell chamber to increase the fuel utilisation, to produce an exhaust gas that contains essentially carbon dioxide and water for further treatment so that carbon dioxide can be separated and is not vented into the atmosphere. However, this system does not utilise the carbon dioxide separation system to increase the electrical efficiency further than what is obtained by the increased fuel utilisation.
“SOFC Efficiency at non standard conditions”, Electrochemical Proceedings Volume 97-18, presents theoretical considerations for SOFC systems with high efficiency, and suggests the circulation of anode gas with condensation of water and recycling of hydrogen as a means for obtaining high efficiencies in hydrogen fuelled systems. Although improved electrical efficiencies may be realised by this theoretical concept, this can only be done by reducing the power density, since a very high cell potential is required. This paper does not disclose systems which exhibit both high efficiency and high power density.
U.S. Pat. No. 2,781,248, BE 881637 and other documents disclose systems for the manufacture of hydrogen using calcium oxide as a carbon dioxide absorbent.
Whilst the use of anode gas recycling to improve the efficiency of a fuel cell system, and the use of calcium oxide/calcium carbonate cycles for the manufacture of hydrogen are known, there remains a need for improvements with respect to electrical efficiency, power density, carbon dioxide separation and parasitic losses.